A number of arrangements are known in the art for coupling signals from a bidirectional transmission path or facility to receive and transmit unidirectional transmission paths or facilities. One example of this type transmission network is employed to couple a two-wire bidirectional telephone transmission facility to a four-wire telephone transmission facility.
As is now known, it is desirable to adjust a complex impedance circuit in the transmission coupling network in order to obtain a better match to the impedance of the bidirectional transmission path or facility, thereby maximizing loss between the receive and transmit unidirectional paths or facilities. This is commonly referred to as maximizing "transhybrid" loss.
Many transmission networks employ so-called hybrid networks to realize the desired bidirectional-to-unidirectional coupling. Transmission networks employing either hybrid transformers or electronic circuits are now commonly employed in telephone transmission systems for this purpose. In using either a hybrid transformer or an electronic "hybrid" it is desirable to employ a network having an impedance which substantially matches the impedance of the bidirectional transmission facility. Otherwise, low transhybrid loss results which, in turn, yields unwanted signal reflections. That is to say, a portion of the signal on the receive unidirectional path or facility appears in the transmit unidirectional path or facility. To this end, in transformer type hybrids, a complex impedance network is employed in an attempt at matching the impedance of the bidirectional path or facility. Similarly, in electronic canceler type "hybrids", a network having a complex transfer (impedance) characteristic is employed to generate a correction signal in attempting to cancel an unwanted error signal appearing as a component of the signal to be transmitted on the transmit unidirectional path or facility.
In either arrangement adjustable impedance networks have been used in order to obtain a better impedance match to the bidirectional facility and, hence, to maximize transhybrid loss.
In transmission networks which employ hybrid transformers, it has become the practice to employ an electronic network to generate a driving point impedance which emulates the complex impedance of the bidirectional transmission facility and, thereby, balance the hybrid transformer. Similarly, in transmission networks which employ canceler arrangements, an active impedance network is employed having a complex transfer characteristic which emulates the impedance characteristic of the bidirectional facility in order to generate the correction signal used for canceling the unwanted error signal in the transmit path or facility. A substantially fixed impedance canceler circuit is disclosed in U.S. Pat. No. 4,074,087 issued to R. B. Blake, Jr., et al., on Feb. 14, 1978.
In a copending application of J. F. Rizzo, Ser. No. 064,042, filed Aug. 6, 1979, now U.S. Pat. No. 4,275,276, issued June 23, 1981, an arrangement is disclosed for automatically adjusting impedance elements of an adjustable impedance network in an attempt at obtaining an optimum match to a particular bidirectional transmission facility. The arrangement employs individual tones, which are supplied to a receive port of a transmission network while corresponding individual impedance elements are adjusted until an amplitude null is detected at a transmit port of the transmission network. The tone signals are supplied and the amplitude adjustments are made in a prescribed sequence in order to obtain the best match to the impedance of the bidirectional facility. Then, a pseudo-random noise signal having predetermined frequency components across the entire voiceband is supplied to the receive port and one of the impedance elements is readjusted. The disclosed procedure rapidly yields an optimum match for a loaded bidirectional facility when adjusting an impedance network intended for use with hybrid transformer coupling arrangements.
However, when employing canceler type impedance circuits, it is extremely important to select the proper variables to adjust in order to obtain an optimum impedance match to the bidirectional facility and the proper adjustment sequence for converging the variables rapidly to the right settings for yielding the optimum match. We have learned from experimentation that the variables, adjustment sequence and signals employed cannot be arbitrarily chosen and still obtain the desired optimum result. Indeed, the wrong choice may result in adjustment of the canceler impedance to generate a transfer function which may not even closely match the bidirectional facility impedance. This would result in undesirable signal reflections.
Additionally, it is now desirable to include a network build out capacitor on the network side of the transmission network as distinguished from a line build out capacitor which is connected across terminals on the bidirectional facility side of the transmission network. Network build out capacitors are desirable because they can be electronically controlled and, therefore, easily adjusted. It is undesirable to include electronically controllable arrangements on the bidirectional facility side of the circuit because of their susceptibility to damage from voltage surges, e.g., lightning, and because of noise insertion in the facility.
Use of such adjustable capacitors makes it desirable to automatically adjust them in the impedance adjustment procedure. However, since the adjustment of the network build out capacitor does affect the impedance match of the bidirectional cable, such adjustment further complicates the adjustment of the transmission network impedance.
The adjustment procedure is further complicated for canceler type transmission networks intended to couple to loaded telephone 2-wire cable because of the use of 25 gauge 2-wire telephone cable commonly referred to as Metropolitan Area Trunk (MAT) cable.